Recently, with reduction on size and weight of instruments, a creep distance acting to the surfaces of insulators of the instruments is greatly increased and it has been required for a pressure-sensitive adhesive tape used by taping as an interlayer electrical insulation of the coil for transformer, etc., to greatly increase tracking resistance properties of the tape.
Hitherto, a polyester pressure-sensitive adhesive tape and a barrier tape (epoxy-impregnated polyester/glass cloth) are used for interlayer electrical insulation of a transformer coil, etc., of home electric appliances, but the tracking resistant properties of these electrical insulating tapes are in the range of from 250 to less than 400 V by the evaluation of a comparative tracking index (hereinafter referred to as "CTI") of an IEC method. However, recently, for increasing the tracking resistant properties, it has been required to increase the CTI value above 400 V.
Now, in an electrically insulating pressure-sensitive adhesive tape which is used by interlayer taping the coil for transformer, etc., it is necessary to use a considerably self-supporting tape in order to avoid poor handling properties of the tape at taping due to flexibility of the tape (hereinafter referred to as "poor handling properties" for simplicity).
However, if the self-supporting property of a pressure-sensitive adhesive tape is increased, the bending rigidity of the tape is increased and the anti-bending force of the taped pressure-sensitive adhesive tape itself is increased, whereby the pressure-sensitive adhesive tape tends to peel off at the taped end portion.
However, since the taped end portion of a pressure-sensitive adhesive tape exists at the surface of the taped layer, which is the place having a possibility of being exposed to a tracking phenomenon, if the taped end portion peels off and lifts from the surface, cutting by a Joule's heat of an electrically conductive liquid(water containing salts, dusts, or ionically contaminated materials)-attached leaked electric current path through the taped end portion tends to occur at the taped end portion. In the instance of cutting, a discharge occurs and since the discharge causes carbonization at the beginning of tracking, peeling off of the taped end of the pressure-sensitive adhesive tape becomes a cause of causing a tracking phenomenon.
Hitherto, for restraining the occurrence of the tracking phenomenon, an electrically insulating material having excellent tracking resistant properties by itself or an electrically insulating material having mixed therewith an inorganic powder has been used.
However, in an electric instrument electrically insulated by taping of an electrically insulating pressure-sensitive adhesive tape, even when a material having excellent tracking resistant properties is used for the electrically insulating tape, if peeling off occurs at the taped end of the electrically insulating pressure-sensitive adhesive tape as described above, the improvement on the tracking resistant properties of the instrument itself cannot be attained.